Electrically Insulating Plastics

Electrically insulating plastics are polymers whose electrons are locked in covalent bonds. With no free charge carriers, no current flows; volume resistivity sits at 10¹² to 10¹⁸ Ω·cm, compared with 10⁻⁶ Ω·cm for copper. Almost every unfilled plastic insulates, the differences lie in four properties: volume resistivity, dielectric strength, permittivity and dissipation factor, plus CTI for the surface and the continuous service temperature. This article places 17 materials side by side with their test standard in one table, separates them from conductive plastics and leads to a selection in six steps. GOBA converts the film materials among them, above all PET, polyimide and PTFE, from its range of insulation materials into slit rolls from 5 to 1,600 mm wide and punched parts from 0.023 to 3.0 mm thick.

• PTFE, PE and PP insulate best in the bulk, polyimide and PEN film withstand the highest breakdown stress (303 kV/mm at 25 µm), polyimide, PTFE and PPS carry the highest temperature.
• PA, PVC and TPU are polar: they absorb water or lose energy in an alternating field and therefore drop out as insulators for precision applications.
• The volume resistivity of a plastic falls by orders of magnitude with temperature and humidity, so a high catalogue value alone is not enough for the design.

Which plastics insulate electrically?

The line sits at a volume resistivity of 10¹¹ Ω·cm; above that value a material counts as an insulator. Nearly every unfilled plastic clears it, from the polyethylene in a cable jacket to the polyimide in a motor: among the thermoplastics PE, PP, PS, PTFE, PET, PEN, PC, POM, PA, PEEK, PPS and polyimide, among the thermosets epoxy and phenolic resin, among the elastomers silicone and polyurethane.

In practice the question is therefore rarely whether a plastic insulates. It is how well it does so at the temperature, field strength, humidity and thickness that actually occur in the part. That is exactly where one electrical insulator made of plastic differs from the next by orders of magnitude. The material groups of electrical insulation as a whole, including papers, mica and laminates, are organised in the hub on insulation material.

Why plastics insulate electrically

A metal conducts because its valence electrons move freely through the lattice. A polymer consists of carbon chains whose electrons are locked in covalent single bonds. The gap between the filled valence band and the empty conduction band, the band gap, amounts to several electronvolts in saturated polymers such as PE or PTFE. At room temperature the thermal energy is nowhere near enough to lift electrons across that gap. There are simply no charge carriers for a field to move. Whatever residual current still flows comes from impurities, ions from catalyst residues and absorbed water.

This leads to the second distinction that matters for selection: polar or non-polar. Non-polar polymers such as PE, PP, PS and PTFE carry no dipoles along the chain. In an alternating field there is nothing that has to align, which is why permittivity sits at 2.1 to 2.5 and the dissipation factor below 0.0005. Polar polymers such as PA, PVC, PET and polyurethane carry amide, chlorine, ester or urethane groups. These dipoles follow the field, heat the material and attract water. Polyamide absorbs 3 percent water in standard climate, and that water lowers resistivity by three orders of magnitude. How conductivity is defined physically and measured in siemens per metre is explained in the article on electrical conductivity.

The properties that describe an insulating plastic

A datasheet for electrical insulating materials carries six electrical lines. Four describe the bulk, two the surface. Each has its own test standard, and without the standard a value cannot be compared.

Volume resistivity says whether a material insulates at all. The dielectric strength says how thin it may be while doing so. Permittivity and dissipation factor say how it behaves in an alternating field, at 50 Hz on the grid, at a few kilohertz in an inverter and at megahertz in signal electronics. The CTI and the surface resistance describe what happens on the surface once dust and moisture arrive. Anyone measuring the finished device tests none of these material values, they measure the insulation resistance of the whole assembly.

Properties of electrically insulating plastics compared

The following table places 17 insulating plastics side by side. The values come from manufacturer datasheets and the RIWETA datasheet collection of Kern GmbH: volume resistivity to IEC 60093, dielectric strength to IEC 60243-1 on a 1 mm specimen, permittivity and dissipation factor to IEC 60250 at 1 MHz, CTI to IEC 60112. Film values are marked as such and apply at the stated thickness. The thermal class is a guide value derived from the continuous temperature against the class limits of IEC 60085; a rating to IEC 60216 exists only for PET film, PEN, polyimide, FR4 and phenolic paper.

All values are guide values for uncoloured standard grades in standard climate. Additives, fillers, pigments and flame retardants change them, sometimes by an order of magnitude. Design work follows the datasheet of the specific grade. The table nevertheless shows four patterns that recur in every material selection.

First, dielectric strength is not a material constant. PP delivers 50 kV/mm on a 1 mm specimen and 200 kV/mm as a 40 µm film, PVC 40 against 150, Kapton HN 303 kV/mm at 25 µm against 154 kV/mm at 125 µm. Thin layers withstand more per millimetre because defects and field enhancements coincide less often. Anyone extrapolating a sheet value linearly onto a film underestimates the film; anyone transferring a film value onto a sheet endangers the part. How this translates into the choice of thickness is covered in the article on material thickness.

Second, polyamide is a good insulator when dry and a mediocre one when moist. Volume resistivity falls from 10¹⁵ to 10¹² Ω·cm, permittivity rises from 3.1 to 4.7 and the dissipation factor from 0.017 to 0.13. Datasheet values for PA apply in the dry, conditioned state. In motor construction and household appliances, where moisture is certain to occur, work with the moist values.

Third, thermal endurance is not the same as electrical strength. PEEK withstands 240 °C but reaches only 25 kV/mm and a CTI of 150. Glass-filled PPS sits at CTI 125. The aromatic rings that make these materials heat resistant char into conductive carbon under the arc of the CTI test. PE, PP, POM and PTFE without rings reach CTI 600 even though they drop out at 90 to 100 °C. What the CTI means for creepage distances is explained in the article on tracking resistance.

Fourth, polarity costs energy in an alternating field. TPU has a respectable resistivity of 10¹³ Ω·cm but a permittivity of 6.0 and a dissipation factor of 0.07. With an inverter switching at 16 kHz, part of the field energy turns into heat inside the insulation in every cycle. PET sits at 0.021, polyimide at 0.0018, PP at 0.0004. For film capacitors and high-frequency assemblies the dissipation factor is therefore the first selection criterion, not resistivity.

Conductive and antistatic plastics: when plastic conducts electricity

Plastic conducts electricity when it is made to, and there are three ways to get there. The first is a filler: conductive carbon black, graphite, carbon fibres or carbon nanotubes are compounded into the matrix until they touch and form a continuous network, the so-called percolation threshold. The second way is a conductive coating or an antistatic agent that migrates to the surface and forms a thin, water-binding film there. The third way is intrinsically conductive polymers such as polyaniline, polypyrrole or PEDOT:PSS, whose conjugated double bonds transport charge once doped. Their conductivity ranges, according to Wikipedia, from 10⁻¹³ to 10³ S/cm, and their discovery earned Heeger, MacDiarmid and Shirakawa the 2000 Nobel Prize in Chemistry.

The limits follow the classification used by stock shape suppliers such as Muovia and Ensinger; the standard IEC 61340-5-1 for ESD protection defines its own test setups. For purchasing there is one consequence: a material name says nothing about conductivity. POM exists as an insulator at 10¹⁶ Ω·cm and as POM ELS with a surface resistance below 10⁴ Ω, and so does PE 1000. Anyone ordering HDPE or PP as an insulating material orders the unfilled natural grade and writes that into the specification. Black colouring is a warning sign here, because black in plastics usually comes from carbon black, and carbon black is the most common conductive additive.

How to select the right insulating plastic

The order of the checks matters more than their number. Anyone who rounds generously in step one cannot recover the error in step six. For web material and insulating parts we recommend these six steps.

1. Voltage and field strength. Determine operating voltage, test voltage and the planned thickness, which gives the field strength in kV/mm. Compare it with the dielectric strength at exactly that thickness, not with the value on a 1 mm specimen, and allow a safety factor. With inverter operation and steep edges, partial discharge becomes a limit of its own.
2. Temperature. Establish the continuous temperature at the hottest point of the insulation, not at the housing, and choose the thermal class with 20 to 30 °C headroom. That rules out PE, PP, PVC, PS, POM, PA and TPU for anything above 100 °C; PET covers class B, PEN class F, silicone and polyimide sit above.
3. Moisture and chemistry. Clarify whether water, impregnating resin, oil, coolant or cleaning agents reach the part. PA drops out under moisture, PET absorbs practically no water, PTFE and PP withstand almost any chemical, silicone swells in oil.
4. Mechanics and processing. An insulating part made of film is slit, punched, creased and slid into a slot. For that the material needs tensile strength, tear propagation resistance and a defined springback. Biaxially oriented PET film delivers that, phenolic paper breaks when creased, PTFE creeps under clamping pressure. Machined parts follow different criteria; that is where PEEK, POM and PC come in.
5. Fire safety. Read the required class to UL 94 and the glow-wire temperature from the equipment standard. PTFE, PPS, PEEK, polyimide and PVC reach V-0 without additives, PE, PP, PET and POM only HB in the natural grade. Flame retardants often lower CTI and dielectric strength, so after step five go back and check step one again.
6. Cost and availability. Only now is the price per square metre worth a look. PET costs a fraction of polyimide, and where class B is enough, the premium for class C is wasted. Where class H or installation space sets the limit, it is the cheapest item in the motor.

For each field of application this order yields a first recommendation. It does not replace a design calculation, but it shortens the list of candidates to two or three materials.

We advise against PVC as sheet insulation in motors. The material withstands 60 °C, contains plasticisers that migrate under heat and releases hydrogen chloride as it decomposes. Which films take its place is shown in the article on the PVC film alternative. Whether a thermoplastic or a thermoset is the better material class is decided by the production form, covered under thermoplastics and thermosets. Fibre-reinforced laminates such as FR4 and laminated fabric are placed by the article on composite materials.

Is the cheapest plastic with a high resistivity not enough?

The objection comes from purchasing and looks sound at first glance: PE-HD has more than 10¹⁶ Ω·cm, CTI 600 and costs less than any other material in the table. So why PET, let alone polyimide? The answer lies in three numbers that volume resistivity does not show.

The first is temperature. PE softens from 90 °C, a class F motor runs at 155 °C. The resistivity of a plastic falls exponentially with rising temperature, and Toray states for PPS that resistivity drops from above 10¹³ to 10¹⁰ Ω·m above the glass transition temperature. A material that shines with 10¹⁶ Ω·cm at 23 °C may sit at 10¹² at the winding. The second number is moisture. Polyamide loses three orders of magnitude to water from the air, and the surface of every plastic turns into a creepage path under condensation and dust. That is where the CTI protects, and it is good for PE and poor for PEEK and FR4. The third number is thickness. Dielectric strength determines how thin the insulation may be, and every tenth of a millimetre in the slot costs copper cross-section. A 0.25 mm PET film replaces a considerably thicker layer of a material with 20 kV/mm.

The cheapest plastic is therefore the one that still holds its values at operating temperature, humidity and target thickness, and in motor construction that is usually PET, above 130 °C aramid or polyimide. The price per square metre is almost never the problem. A slot liner costs cents inside a motor; a winding failure in the field costs the teardown.

Which insulating plastics GOBA converts

GOBA has been processing the film and laminate materials from the table since 1959, not the machined engineering plastics. We do not supply PEEK, PPS, POM or PC as blocks or injection moulded parts, but we do supply everything that comes onto a roll as a web and blocks voltage as a thin layer in a motor, transformer, battery module or household appliance. The core is biaxially oriented polyester film under the brands Hostaphan and Mylar in thermal class B up to 130 °C to IEC 216; for Hostaphan heavy-gauge films we are the largest European converter. These polyester films go into winding machines as slit rolls from 5 mm wide and into stators as punched parts.

• Polyester film PET in class B, plasticiser free, with practically no water absorption, as roll stock, cut sheet, punched part and moulded part.
• Polyimide film Kapton in class C with 303 kV/mm at 25 µm, alone or as the middle layer in NKN laminate for class H.
• PTFE film for chemically and thermally stressed separating layers and gaskets.
• PE, PP and PU films for protective, separator and damping tasks, including PE and PP separators and cell insulation films in battery film slitting.
• Laminates of film with aramid paper or nonwoven, such as DMD, NMN and NKN, for class B to H.

All grades are UL listed, and production runs to ISO 9001 and ISO 14001. In contract slitting, two circular blades running past each other shear the web into widths from 5 to 1,600 mm with a width tolerance of plus minus 0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inner diameter, at cutting speeds up to 400 m per minute and parent rolls up to 1,000 kg. In stamping and forming we process material thicknesses from 0.023 to 3.0 mm into parts up to 1,000 x 2,000 mm, with steel rule dies for standard contours and solid steel tooling for tight tolerances. We pre-emboss fold lines in the same stroke so that the springback of the film does not work against assembly, and we form complex geometries under heat.

From the production side, one detail matters with insulating plastics that no datasheet mentions: the values apply to the undamaged film. A frayed cut edge or a crack along the fold line is the spot where the field peaks and breakdown begins. That is why we run thin films slower and inspect the edge as closely as the width. Which materials and classes we carry overall is listed under insulation materials.

GOBA Takeaway

Electrically insulating plastics differ less in whether they insulate than in how. Almost every unfilled plastic has 10¹² Ω·cm or more. Selection comes down to dielectric strength at the target thickness, continuous temperature, behaviour under moisture and in an alternating field, and the CTI of the surface. For sheet insulation that means PET up to class B in most cases, PEN or laminates up to class F, aramid and polyimide above, and PP or PTFE wherever the dissipation factor tips the balance.

If you need a material from the table as a slit roll or insulating part, send us the drawing with voltage level, temperature and thickness. We check material choice and production form in one pass and deliver from our insulation materials from sample to series.

FAQ on Electrically Insulating Plastics

Which plastic insulates best?

That depends on the property. For volume resistivity PTFE leads at 10¹⁸ Ω·cm, followed by polyimide film at 1.5 × 10¹⁷ and PE, PP and POM at 10¹⁶ Ω·cm. For dielectric strength the thin films lead, Kapton HN at 303 kV/mm at 25 µm and PEN at 300 kV/mm at 12 µm. For temperature PTFE leads at 260 °C, PEEK at 240 °C and polyimide at 230 °C. In motor construction PET usually wins because it combines dielectric strength, class B, creasability and price.

Is plastic an electrical insulator?

Yes, almost every unfilled plastic is an electrical insulator with a volume resistivity above 10¹¹ Ω·cm, because its electrons are bound in covalent bonds and no free charge carriers exist. The exceptions are grades deliberately made conductive with carbon black, carbon fibre or intrinsically conductive polymers. For electrical insulation you therefore order the unfilled natural grade.

Does plastic conduct electricity?

Normally not measurably; at 10¹² to 10¹⁸ Ω·cm the residual current sits more than twenty orders of magnitude below copper. Plastic conducts electricity only once conductive carbon black, graphite, carbon fibres or metal fibres are mixed in beyond the percolation threshold, or in intrinsically conductive polymers such as polyaniline and PEDOT:PSS. Antistatic grades sit at 10⁹ to 10¹¹ Ω, dissipative ESD grades at 10⁶ to 10⁹ Ω and conductive grades below 10⁴ to 10⁶ Ω surface resistance.

What dielectric strength do plastics have?

On a 1 mm specimen to IEC 60243-1 most plastics sit between 15 and 55 kV/mm: PA 6 at 15, PTFE at 20, PET and PEEK at 25, PC at 34, PVC and POM at 40, PS at 43, PP at 50, PE-HD at 53 kV/mm. Thin films sit several times higher, PP at 200 kV/mm at 40 µm, PET up to around 180 kV/mm, polyimide at 303 kV/mm at 25 µm. The value must therefore always be read together with the test thickness.

Is PVC a good electrical insulator?

For cable jackets and core insulation up to 60 to 70 °C, yes: PVC-U reaches 10¹⁵ Ω·cm, 40 kV/mm and CTI 600 and is inherently flame retardant at UL 94 V-0. As sheet insulation in a motor or transformer PVC is unsuitable, because its continuous temperature ends at 60 °C, its dissipation factor of 0.03 is high, plasticisers migrate under heat and hydrogen chloride forms on decomposition. There PET film replaces PVC.

What does CTI 600 mean for a plastic?

CTI 600 means that in the test to IEC 60112 the surface survives 50 drops of electrolyte solution at 600 volts without forming a conductive track. That is the highest value on the standard scale and places the material in material group I of IEC 60664-1, which permits the shortest creepage distances. PE, PP, POM, PTFE, PVC, PA, TPU and silicone reach CTI 600, PET sits at 350, PC at 250, PEEK at 150 and FR4 at 175 to 249.